diff --git a/features/doc-add-comment.feature b/features/doc-add-comment.feature
index 36f46244a..332ea6f7b 100644
--- a/features/doc-add-comment.feature
+++ b/features/doc-add-comment.feature
@@ -11,3 +11,8 @@ Feature: Add a comment to a document
       And comment.text == "A comment"
       And comment.author == "John Doe"
       And comment.initials == "JD"
+
+  Scenario: Document.add_comment() registers the CommentReference character style
+    Given a new document without a CommentReference style
+     When I assign comment = document.add_comment(runs, "A comment", "John Doe", "JD")
+     Then the document styles include a "CommentReference" character style
diff --git a/features/steps/comments.py b/features/steps/comments.py
index 39680f257..2f6aa13e9 100644
--- a/features/steps/comments.py
+++ b/features/steps/comments.py
@@ -45,6 +45,12 @@ def given_a_document_having_no_comments_part(context: Context):
     context.document = Document(test_docx("doc-default"))
 
 
+@given("a new document without a CommentReference style")
+def given_a_new_document_without_a_comment_reference_style(context: Context):
+    context.document = Document()
+    context.document.add_paragraph("Test paragraph for comment anchor.")
+
+
 # when =====================================================
 
 
@@ -282,3 +288,15 @@ def then_the_result_is_a_comment_object_with_id_2(context: Context):
     comment = context.comment
     assert type(comment) is Comment, f"expected a Comment object, got {type(comment)}"
     assert comment.comment_id == 2, f"expected comment_id `2`, got '{comment.comment_id}'"
+
+
+@then('the document styles include a "CommentReference" character style')
+def then_document_styles_include_comment_reference_style(context: Context):
+    from docx.enum.style import WD_STYLE_TYPE
+
+    styles = context.document.styles
+    style = styles._element.get_by_id("CommentReference")
+    assert style is not None, "expected 'CommentReference' style in styles.xml, but it is absent"
+    assert style.type == WD_STYLE_TYPE.CHARACTER, (
+        f"expected CHARACTER style, got {style.type}"
+    )
diff --git a/src/docx/document.py b/src/docx/document.py
index 73757b46d..108994aa3 100644
--- a/src/docx/document.py
+++ b/src/docx/document.py
@@ -79,6 +79,9 @@ def add_comment(
         first_run = runs[0]
         last_run = runs[-1]
 
+        # -- guarantee the 'CommentReference' character style is defined in styles.xml --
+        self.styles._ensure_comment_reference_style()
+
         # -- Note that comments can only appear in the document part --
         comment = self.comments.add_comment(text=text, author=author, initials=initials)
 
diff --git a/src/docx/styles/styles.py b/src/docx/styles/styles.py
index b05b3ebb1..f69bc070d 100644
--- a/src/docx/styles/styles.py
+++ b/src/docx/styles/styles.py
@@ -6,7 +6,7 @@
 
 from docx.enum.style import WD_STYLE_TYPE
 from docx.oxml.styles import CT_Styles
-from docx.shared import ElementProxy
+from docx.shared import ElementProxy, Pt
 from docx.styles import BabelFish
 from docx.styles.latent import LatentStyles
 from docx.styles.style import BaseStyle, StyleFactory
@@ -115,6 +115,31 @@ def _get_by_id(self, style_id: str | None, style_type: WD_STYLE_TYPE):
             return self.default(style_type)
         return StyleFactory(style)
 
+    def _ensure_comment_reference_style(self) -> None:
+        """Add the built-in 'CommentReference' character style if not present.
+
+        `Document.add_comment()` writes `<w:rStyle w:val="CommentReference"/>` into the
+        reference run but never adds the corresponding style definition to styles.xml.
+        Word silently tolerates the dangling reference, but the reference mark then
+        inherits default run formatting instead of the intended 8-pt "annotation
+        reference" appearance.  This method adds the style once per document, skipping
+        the work on subsequent calls.
+        """
+        if self._element.get_by_id("CommentReference") is not None:
+            return
+        style_elm = self._element.add_style_of_type(
+            "annotation reference", WD_STYLE_TYPE.CHARACTER, True
+        )
+        # The auto-generated styleId from the name would be "annotationreference";
+        # override it to match the id that the reference run already uses.
+        style_elm.styleId = "CommentReference"
+        style_elm.basedOn_val = "DefaultParagraphFont"
+        style_elm.uiPriority_val = 99
+        style_elm.semiHidden_val = True
+        style_elm.unhideWhenUsed_val = True
+        rPr = style_elm.get_or_add_rPr()
+        rPr.sz_val = Pt(8)
+
     def _get_style_id_from_name(self, style_name: str, style_type: WD_STYLE_TYPE) -> str | None:
         """Return the id of the style of `style_type` corresponding to `style_name`.